[FIX] ir_qweb: decoding of attachment assets

For uploaded modules (e.g. saas), uploaded assets are stored in
attachments.

When the asset content is retrieved, static "filesystem" files are
assumed to be UTF8 and properly decoded, but attachment data is not
and is thus interpreted as ASCII further down the line, leading to a
UnicodeDecodeError (and a broken page/template/...) if the asset data
is not actually ASCII (e.g. if it's UTF-8).

Fix by treating attachment assets as UTF8 encoded, same as filesystem
assets.

OPW-752858

odoo/addons/base/ir/ir_qweb/assetsbundle.py

@@ -479,7 +479,7 @@ class WebAsset(object):
                 with open(self._filename, 'rb') as fp:
                     return fp.read().decode('utf-8')
             else:
-                return self._ir_attach['datas'].decode('base64')
+                return self._ir_attach['datas'].decode('base64').decode('utf-8')
         except UnicodeDecodeError:
             raise AssetError('%s is not utf-8 encoded.' % self.name)
         except IOError: